Keeping in mind that we're playing with Admin which is misbehaving, you should back up your database before tinkering...in case something goes wrong.
The greyed out boxes indicate that the permission is inherited from another group. Perhaps someone tried to "include" the group Admin into Admins. Look at the top of the Edit Permissions page for Admins, where there is a red "X" next to the groups which are being inherited. If there is a red "X" next to the group Admin, click on the X to remove Admin from Admins.